Chapter Summary

• A multicellular organism begins its development as an embryo. Embryogenesis results in a new organism with a body plan characteristic of its species. Review Figure 19.1, Activity 19.1

• The processes of development are determination, differentiation, morphogenesis, and growth.

• Differential gene expression is responsible for the differences among cell types. Cell fate is determined by environmental factors, such as the cell’s position in the embryo, as well as by intracellular influences. Review Figure 19.2, Activity 19.2

• Over the course of development, embryo cells decrease in cell potency. Totipotent cells (such as a zygote) are capable of forming every cell type in the adult body. Pluripotent cells can give rise to most cell types, multipotent cells to several cell types, and unipotent cells to only one cell type.

• The ability to create clones from differentiated cells demonstrates the principle of genomic equivalence. Review Figures 19.3, 19.4

• Stem cells produce daughter cells that differentiate when provided with appropriate intercellular signals. Some multipotent stem cells in the adult body can differentiate into a limited number of cell types to replace dead cells and maintain tissues. Review Investigating Life: Stem Cell Therapy

• Embryonic stem cells (ESCs) are pluripotent and can be cultured in the laboratory. Under suitable environmental conditions, these cells can differentiate into almost any tissue type. Induced pluripotent stem cells (iPS cells) have similar characteristics, making possible technologies designed to replace cells or tissues damaged by injury or disease. Review Figure 19.5, Animation 19.1

• Cytoplasmic segregation—the unequal distribution of cytoplasmic determinants in the egg, zygote, or early embryo—can establish polarity and lead to cell fate determination. Review Figure 19.6, Animation 19.2

• Induction is a process by which embryonic animal tissues direct the development of neighboring cells and tissues by secreting chemical signals, called inducers. Review Figure 19.7

• Inducers act through signaling pathways to determine cell fate. Review Focus: Key Figure 19.8

• Differential gene expression results in cell differentiation. Transcription factors are especially important in regulating gene expression during differentiation. Review Figure 19.9

• Pattern formation is the process that results in the spatial organization of a tissue or organism.

• Both plants and animals use positional information as a basis for pattern formation. Positional information usually comes in the form of a signal called a morphogen. Different concentrations of the morphogen cause different effects. Review Figure 19.10

• Sepals, petals, stamens, and carpels form in plants as a result of combinatorial interactions between transcription factors encoded by organ identity genes. Review Figure 19.11, Activity 19.3

• In the fruit fly Drosophila melanogaster, a cascade of transcriptional activation sets up the axes of the embryo, the development of the segments, and finally the determination of cell fate in each segment. The cascade involves the sequential expression of maternal effect genes, gap genes, pair rule genes, segment polarity genes, and Hox genes. Review Figures 19.12, 19.13, Animation 19.3

• Hox genes help determine cell fate in the embryos of all animals. The homeobox is a DNA sequence found in Hox genes and other genes that code for transcription factors. The sequence of amino acids encoded by the homeobox is called the homeodomain. Review Figure 19.14

• Evolutionary developmental biology, or evo-devo, is the study of the evolutionary aspects of development. This field focuses on the molecular mechanisms that underlie the development of phenotypic diversity.

• Similarities in the basic mechanisms of development between widely divergent organisms reflect common ancestry.

• Genes encoding transcription factors and other regulatory proteins that govern pattern formation in the developing bodies of multicellular organisms comprise what is called a genetic toolkit. These regulatory genes have been highly conserved throughout evolution. Review Figure 19.15

• The bodies of developing and mature organisms are organized into self-contained units, or modules, that can be modified independently. See Animation 19.4

• The genetic toolkit involves genetic switches—promoters, enhancers and repressors, signaling molecules, and signal transduction components—that can alter the expression of developmental genes in different modules independently of one another. Review Figure 19.16

• Developmental genes can be expressed in a modular fashion in different amounts (heterometry), at different times (heterochrony), or in different locations (heterotopy). Review Figures 19.17–19.19

• Morphological differences among species can result from mutations in the genes that regulate the development of modules such as body segments or wings. Review Figure 19.20

• Because many genes that govern development have been highly conserved, similar traits are likely to evolve repeatedly, especially among closely related species. This process is called parallel evolution. Review Figure 19.21